Searcharxiv⌕ Search

arXiv subjects

Tom Bachmann

Publications and source records attributed to Tom Bachmann.

46 records · Page 3Linked to original sources

Towards conservativity of $\mathbb{G}_m$-stabilization

We study the interplay of the homotopy coniveau tower, the Rost-Schmid complex of a strictly homotopy invariant sheaf, and homotopy modules. For a strictly homotopy invariant sheaf $M$, smooth $k$-scheme $X$ and $q \geqslant 0$ we construct a novel cycle complex $C^*(X, M, q)$ and we prove that in favorable cases, $C^*(X, M, q)$ is equivalent to the homotopy coniveau tower $M^{(q)}(X)$. To do so we establish moving lemmas for the Rost-Schmid complex. As an application we deduce a cycle complex model for Milnor-Witt motivic cohomology. Furthermore we prove that if $M$ is a strictly homotopy invariant sheaf, then $M_{-2}$ is a homotopy module. Finally we conjecture that for $q>0$, $\underlineπ_0(M^{(q)})$ is a homotopy module, explain the significance of this conjecture for studying conservativity properties of the $\mathbb{G}_m$-stabilization functor $\mathcal{SH}^{S^1}\!(k) \to \mathcal{SH}(k)$, and provide some evidence for the conjecture.

math.AG↗

Affine Grassmannians in A^1-algebraic topology

Let k be a field. Denote by Spc(k)_* the unstable, pointed motivic homotopy category and by Omega_Gm: Spc(k)_* \to Spc(k)_* the Gm-loops functor. For a k-group G, denote by Gr_G the affine Grassmannian of G. If G is isotropic reductive, we provide a canonical motivic equivalence Omega_Gm G = Gr_G. If k is perfect, we use this to compute the motive M(Omega_Gm G) in DM(k, Z).

math.AG↗

A1-invariants in Galois cohomology and a claim of Morel

We establish a variant of the splitting principle of Garibaldi-Merkurjev-Serre for invariants taking values in a strictly homotopy invariant sheaf. As an application, we prove the folklore result of Morel that pi_0 of the motivic localization of the group completion of the stack of finite étale schemes is given by the sheaf of unramified Grothendieck-Witt groups.

math.KT↗

On the effectivity of spectra representing motivic cohomology theories

Let k be an infinite perfect field. We provide a general criterion for a spectrum in the stable homotopy category over k to be effective, i.e. to be in the localizing subcategory generated by the suspension spectra of smooth schemes. As a consequence, we show that two recent versions of generalized motivic cohomology theories coincide.

math.KT↗

Motivic and Real Etale Stable Homotopy Theory

Let X be a Noetherian scheme of finite dimension and denote by rho the (additive inverse of the) morphism in SH(X) from S to Gm corresponding to the unit -1. Here SH(X) denotes the motivic stable homotopy category. We show that the category obtained by inverting rho in SH(X) is canonically equivalent to the (simplicial) local stable homotopy category of the site X_ret, by which we mean the small real etale site of X, comprised of etale schemes over X with the real etale topology. One immediate application is that SH(RR)[rho^-1] is equivalent to the classical stable homotopy category. In particular this computes all the stable homotopy sheaves of the rho-local sphere (over RR). As further applications we improve a result of Ananyevskiy-Levine-Panin, reprove a vanishing result of Roendigs and establish some new rigidity results.

math.KT↗

On the Conservativity of the Functor Assigning to a Motivic Spectrum its Motive

Given a 0-connective motivic spectrum $E \in SH(k)$ over a perfect field k, we determine $h_0$ of the associated motive $M E \in DM(k)$ in terms of $π_0 (E)$. Using this we show that if k has finite 2-étale cohomological dimension, then the functor M is conservative when restricted to the subcategory of compact spectra, and induces an injection on Picard groups. We extend the conservativity result to fields of finite virtual 2-étale cohomological dimension by considering what we call "real motives". As a by-product we reprove a variant of a rigidity Theorem of Röndings-Østvær.

math.KT↗

Some Remarks on Units in Grothendieck-Witt Rings

We establish new structures on Grothendieck-Witt rings, including a GW(k)-module structure on the unit group GW(k)^x and a presentation of \ul{GW}^x as an infinite Gm-loop sheaf. Even though our constructions are motivated by speculations in stable A1-homotopy theory, our arguments are purely algebraic.

math.KT↗

The Generalized Slices of Hermitian K-Theory

We compute the generalized slices (as defined by Spitzweck-Østvær) of the motivic spectrum KO (representing hermitian K-theory) in terms of motivic cohomology and (a version of) generalized motivic cohomology, obtaining good agreement with the situation in classical topology and the results predicted by Markett-Schlichting. As an application, we compute the homotopy sheaves of (this version of) generalized motivic cohomology, which establishes a version of a conjecture of Morel.

math.KT↗

On the Invertibility of Motives of Affine Quadrics

We show that the reduced motive of a smooth affine quadric is invertible as an object of the triangulated category of motives DM(k, ZZ[1/e]) (where k is a perfect field of exponential characteristic e). We also establish a motivic version of the conjectures of Po Hu on products of certain affine Pfister quadrics. Both of these results are obtained by studying a novel conservative functor on (a subcategory of) DM(k, ZZ[1/e]), the construction of which constitutes the main part of this work.

math.KT↗